Display device

ABSTRACT

A display device includes a substrate; a plurality of light-emitting elements on the substrate; and a plurality of pixel circuits on the substrate, being configured to control the plurality of light-emitting elements in one-to-one correspondence. Each of the plurality of pixel circuits includes a thin film transistor. The thin film transistor includes a channel. The plurality of pixel circuits are disposed at different positions in a scanning direction of a pulse laser beam for annealing the channels. At least channels for light-emitting elements of the same color out of the channels are disposed at the same phase of irradiation cycles of the pulse laser beam in the scanning direction.

CROSS-REFERENCE TO RELATED APPLICATIONS

This Non-provisional application claims priority under 35 U.S.C. §119(a) on Patent Application No. 2018-123598 filed in Japan on Jun. 28,2018 and Patent Application No. 2018-123600 filed in Japan on Jun. 28,2018, the entire contents of which are hereby incorporated by reference.

BACKGROUND

This disclosure relates to a display device. An organic light-emittingdiode (OLED) element is a self-light-emitting element to be driven byelectric current and therefore, it does not require backlight. Inaddition to this, the OLED display element has advantages to achieve lowpower consumption, wide viewing angle, and high contrast ratio; it isexpected to contribute to development of flat panel display devices.

An active matrix type of OLED display device has a display region wherea plurality of pixels are arrayed in columns and rows like a matrix.Each pixel includes one or more subpixels. In the case where each pixelincludes a plurality of subpixels, the subpixels in a pixel emitdifferent colors of light. A subpixel includes a pixel circuit includinga transistor for selecting the subpixel and a driving transistor forsupplying electric current to the OLED element that produces display ofthe subpixel. The transistors included in an OLED display device arethin film transistors (TFTs) and typically, they are low-temperaturepoly-silicon (LTPS) TFTs.

A single-color OLED display device has an array of pixels of a singlecolor only; in contrast, a full-color OLED display device has an arrayof subpixels of three primary colors of red (R), green (G), and blue (B)or an array of white (W) subpixels with RGB color filter arrays toattain full-color display.

To produce poly-silicon including active layers of TFTs, alow-temperature poly-silicon process crystalizes (poly-crystalizes) anamorphous silicon (a-Si) film with an excimer laser annealing (ELA)system. An ELA system is a pulse laser system that irradiates a longnarrow area per shot. The ELA system crystalizes the entire silicon filmon a substrate with the long narrow irradiation area. Accordingly, theELA system scans the substrate in one direction by moving theirradiation area of a pulse laser beam little by little in such a mannerthat the irradiation area of the next shot overlaps the irradiation areaof the previous shot. For this reason, the poly-silicon film has cycliccharacteristics variation in accordance with the scanning pitchdetermined by the pulse frequency and the scanning speed.

Since the subpixels are regularly disposed in rows and columns withinthe display region, the subpixel pitch is usually determined by thescreen size and the resolution. The ELA scanning pitch is determinedfrom the standpoint of the process to attain the fundamentalcharacteristics of the TFTs. Accordingly, the positional relations ofthe TFTs (the channels thereof) to the irradiation areas of successiveshots of a pulse laser beam are different among the subpixels physicallydisposed at different locations in the scanning direction. Hence, theTFTs may have different characteristics among pixel circuits.

For example, in the case where the display region is scanned verticallywith a long narrow pulse laser beam for ELA under the condition wherethe long axis of the pulse laser beam is made coincide with thehorizontal direction of the display region, a displayed image could havecyclic patterns of horizontal bright and dark stripes. These horizontalstripes are called display unevenness. The striped unevenness cyclicallyappearing in a displayed image is caused by the non-uniformity of theTFT characteristics. The display unevenness caused by the non-uniformityof the TFT characteristics could also be observed on a liquid crystaldisplay device as disclosed in US Pat. No. 5,981,974, for example.

SUMMARY

An aspect of this disclosure is a display device including: a substrate;a plurality of light-emitting elements on the substrate; and a pluralityof pixel circuits on the substrate, being configured to control theplurality of light-emitting elements in one-to-one correspondence. Eachof the plurality of pixel circuits includes a thin film transistor. Thethin film transistor includes a channel. The plurality of pixel circuitsare disposed at different positions in a scanning direction of a pulselaser beam for annealing the channels. At least channels forlight-emitting elements of the same color out of the channels aredisposed at the same phase of irradiation cycles of the pulse laser beamin the scanning direction.

Another aspect of this disclosure is a display device including: asubstrate; a plurality of light-emitting elements on the substrate; anda plurality of pixel circuits on the substrate, being configured tocontrol the plurality of light-emitting elements in one-to-onecorrespondence. Each of the plurality of pixel circuits includes a thinfilm transistor. The thin film transistor includes a channel. Thechannel is composed of sections extending in a first direction andsections extending in a second direction, an absolute value of an anglebetween the first direction and a scanning direction of a pulse laserbeam for annealing the channel is a predetermined value, the seconddirection is perpendicular to the scanning direction, and the sectionsextending in the first direction and sections extending in the seconddirection are connected alternately. At least one end of each sectionextending in the second direction is connected with a side extendingalong the first direction of an end part in the first direction of asection extending in the first direction. In each section extending inthe second direction, a middle line extending straight in the seconddirection from one end to the other end is defined as first virtualline. At each position in the first direction of the first virtuallines, a sum of a total length of the first virtual lines and a productof a number of sections extending in the first direction that exist inthe second direction and a channel width takes the same value. At aposition in the first direction where a second virtual line extendingstraight in the second direction does not cross any section extending inthe second direction, a product of a number of sections extending in thefirst direction that is crossed by the second virtual line and thechannel width takes a value equal to the same value. A dimension in thescanning direction of the channel is an integral multiple of a scanningpitch of the pulse laser beam.

It is to be understood that both the foregoing general description andthe following detailed description are exemplary and explanatory and arenot restrictive of this disclosure.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1A schematically illustrates laser annealing with a pulse laserbeam from an ELA system;

FIG. 1B is a graph representing the energy distribution of a pulse laserline beam in the short-axis direction;

FIG. 1C is a first diagram for schematically illustrating the enlargedfront end in the scanning direction of a pulse laser beam;

FIG. 1D is a second diagram for schematically illustrating the enlargedfront end in the scanning direction of a pulse laser beam;

FIG. 1E schematically illustrates an example of the positional relationsof the channels of TFTs to a poly-silicon film having characteristicsvariations in accordance with the irradiation cycle of a pulse laserbeam;

FIG. 2 schematically illustrates a configuration example of an OLEDdisplay device;

FIG. 3A illustrates a configuration example of a pixel circuit;

FIG. 3B illustrates another configuration example of a pixel circuit;

FIG. 4 schematically illustrates a cross-sectional structure of a partof an OLED display device that includes driving TFTs;

FIG. 5 illustrates still another configuration example of a pixelcircuit;

FIG. 6 is a plan diagram of an example of the layout of the elements ofa pixel circuit having the circuit configuration illustrated in FIG. 5;

FIG. 7 schematically illustrates a positional relation among pixelsdisposed in the scanning direction of a pulse laser beam, channels ofthe driving TFTs in the pixels, and the successive irradiation points(lines) of a pulse laser beam;

FIG. 8 provides examples of the combination of N, n, and P_(ELA) thatsatisfies the relation of N×P_(ELA)=n×P_(PIX);

FIG. 9 provides a layout in the conditions where the pixel pitch P_(PIX)is 103.5 μm, the scanning pitch P_(ELA) is 18 μm, the number of pixelsper pixel unit n is 4, and the number of ELA cycles N is 23;

FIG. 10 provides numerical values for specifying the layout provided inFIG. 9;

FIG. 11 illustrates the positions in pixel circuits of the channels inthe layout provided in FIG. 9;

FIG. 12 provides a layout in the conditions where the pixel pitchP_(PIX) is 103.5 μm, the scanning pitch P_(ELA) is 23 μm, the number ofpixels per pixel unit n is 2, and the number of ELA cycles N is 9;

FIG. 13 provides numerical values for specifying the layout provided inFIG. 12;

FIG. 14 illustrates the positions in pixel circuits of the channels inthe layout provided in FIG. 12;

FIG. 15 provides a layout in the conditions where the pixel pitchP_(PIX) is 103.5 μm, the scanning pitch P_(ELA) is 21 μm, the number ofpixels per pixel unit n is 14, and the number of ELA cycles is 69;

FIG. 16 provides numerical values for specifying the layout provided inFIG. 15;

FIG. 17 illustrates the positions in pixel circuits of the channels inthe layout provided in FIG. 15;

FIG. 18 schematically illustrates an example of the positional relationof a bending channel to irradiation lines of a pulse laser beam;

FIG. 19 illustrates a bending channel and a distribution of areasoccupied by the channel at each position in the scanning direction;

FIG. 20 illustrates another bending channel and a distribution of areasoccupied by the channel at each position in the scanning direction;

FIG. 21 is a diagram for illustrating the details of the shape of thechannel illustrated in FIG. 20;

FIG. 22 illustrates another example of the shape of a channel;

FIG. 23 illustrates a relation between a channel having R corners and achannel having square corners;

FIG. 24 illustrates still another example of the shape of a channel;

FIG. 25A illustrates an example of the shape of a channel where theangle between the first direction and the scanning direction is 3degrees;

FIG. 25B illustrates an example of the shape of a channel where theangle between the first direction and the scanning direction is 10degrees; and

FIG. 25C illustrates an example of the shape of a channel where theangle between the first direction and the scanning direction is 20degrees.

EMBODIMENTS

Hereinafter, embodiments of this invention will be described withreference to the accompanying drawings. It should be noted that theembodiments are merely examples to implement this invention and are notto limit the technical scope of this invention.

Overview

In the organic light-emitting diode (OLED) display device to bedisclosed hereinafter, the pixel circuit of a subpixel includes a thinfilm transistor (TFT) for selecting the subpixel and a driving TFT forsupplying electric current to the subpixel. The TFTs in the pixelcircuit are poly-silicon (poly-Si) TFTs having a poly-silicon channel.

The poly-silicon of the TFTs is so-called low-temperature poly-silicon(LTPS). The poly-silicon active layer of a TFT is formed by crystalizing(poly-crystalizing) an amorphous silicon (a-Si) film by scanning it witha pulse laser beam shaped like a long narrow line and further,processing it by photolithography and etching.

The pulse laser system for the crystallization is usually an excimerlaser system, which is also called an excimer laser annealing (ELA)system. FIG. 1A schematically illustrates laser annealing with a pulselaser beam 50 from an ELA system. The pulse laser beam 50 is a longnarrow line beam. When the length of the line is defined as long axisand the width of the line as short axis, the short-axis is parallel tothe scanning direction and the long-axis is perpendicular to thescanning direction. The short-axial width 53 in FIG. 1A is the dimensionalong the short axis of an area irradiated with the pulse laser beam 50and the long-axial length 55 is the dimension along the long axis of thearea irradiated with the pulse laser beam 50.

The ELA system repeatedly irradiates an amorphous silicon film 47 with apulse laser beam while moving the irradiation area little by little insuch a manner that the irradiation area of the next shot overlaps theirradiation area of the previous shot. A typical ELA system changes theirradiation area by sliding a substrate 49. The amorphous silicon film47 instantly melts in response to a pulse laser beam and subsequentlysolidifies into a crystal. The entire amorphous silicon film 47 can becrystalized by being scanned with a long narrow pulse laser beam 50.However, when seen microscopically, the trace of the pulse laser beam 50remains at the end of the short-axial width of each irradiation area onthe crystallized poly-silicon film 40. For this reason, the poly-siliconfilm 40 has cyclic characteristics variation in accordance with thescanning pitch (irradiation pitch) of the pulse laser beam.

The process of crystallization is described more with reference to FIGS.1B to 1E. FIG. 1B is a graph representing the energy distribution of apulse laser line beam in the short-axis direction. In FIG. 1B, thehorizontal axis represents the coordinate in the short-axis directionand the vertical axis represents energy density. Although the idealenergy distribution in the short-axis direction of a pulse laser linebeam is schematically expressed as a rectangle (the dashed line 501 inFIG. 1B), the actual energy distribution has finite inclines at the ends(the solid line 502 in FIG. 1B).

FIGS. 1C and 1D illustrate the process of crystallization of apoly-silicon film 40 in response to a pulse laser beam 50. FIGS. 1C and1D are drawn as if the substrate 49 is fixed and the irradiation area ofthe pulse laser beam 50 moves leftward (in the direction indicated bythe arrow in FIG. 1C). FIG. 1C illustrates a state where a given s-thshot is hitting the substrate 49. The traces of the (s-1)th and priorshots are omitted in FIG. 1C. The forward (left in FIG. 1C) part in thescanning direction of the silicon film is in an amorphous state (47 r)and the area Crs_S irradiated with the s-th shot of the pulse laser beam50 is crystalized. The region Crs41 irradiated with the first range ofthe pulse laser beam 50 are uniformly crystalized. The first range ofthe pulse laser beam 50 corresponds to the region in FIG. 1B where theenergy density is even or highest in the middle of the pulse laser beam.

However, the region Crs42 irradiated with the second range of the pulselaser beam 50 is crystalized differently from the region Crs41. Thesecond range of the pulse laser beam 50 corresponds to the anteriorregion in FIG. 1B where the energy density decreases with incline. Theregion Crs43 irradiated with the third range of the pulse laser beam 50is not crystalized sufficiently. The third range of the pulse laser beam50 corresponds to the front-end region in FIG. 1B where the energydensity falls under the threshold for crystallization. In the regionahead (more left in FIG. 1C) of the region Crs 43, the poly-silicon film40 remains in the amorphous state (47 r). In this way, a trace of thes-th shot of the pulse laser beam is left at the front border of theirradiated area of the poly-silicon film 40.

FIG. 1D illustrates a state of the same part at the (s+1)th shot. TheELA system moves the pulse laser beam 50 in such a manner that the areato be irradiated overlaps the area irradiated previously. Accordingly,in FIG. 1D, the area Crs_S+1 irradiated with the (s+1)th shot of thepulse laser beam 50 is crystalized. The same phenomenon that occurred atthe s-th shot occurs in the front border in the scanning direction. Theend of the irradiation trace generated at the s-th shot is irradiatedwith the first range of the (s+1)th shot of the pulse laser beam 50.However, the process by the laser beam light is a phenomenon that occurswhen the energy of light is absorbed by the workpiece and converts intoheat. When the optical characteristics of the workpiece are not uniform,the generated thermal energy is not uniform, even if the irradiationenergy is uniform.

In the situation of FIG. 1D, the optical characteristics are not uniformdepending on the crystallized state of the poly-silicon film 40. Siliconshows a higher absorptance in its amorphous state than inpolycrystalline state (poly-silicon) for the wavelength of the pulselaser beam commonly used in ELA. Accordingly, the trace of the s-th shotgenerated in the front border in the scanning direction or thedifferently crystalized region is not equalized by the (s+1)th shot butremains. The same applies to the (s+2)th and subsequent shots. Thisdifference in crystalized state cannot be equalized, so that thepoly-silicon film 40 have regions in this differently crystalized stateat regular intervals. In forming TFTs on such a poly-silicon film 40,the difference in crystalized state of the poly-silicon film 40 couldcause differences in transistor characteristics depending on the regionwhere each TFT is formed.

Irradiation points (lines) of a pulse laser beam exist cyclically atequal intervals determined by the pulse frequency and the scanningspeed. When an irradiation line is defined as the front end in thescanning direction of the irradiation area, the TFTs in pixel circuitsmay have slight differences in TFT characteristics depending on theirrelative positions to irradiation lines.

FIG. 1E schematically illustrates an example of the positional relationsof the channels 45 of TFTs to a poly-silicon film 40 havingcharacteristics variation in accordance with the irradiation cycle(spatial cycle) of a pulse laser beam. In the example of FIG. 1E, theTFTs are disposed in such a manner that the scanning direction 51 of thepulse laser beam is parallel to the direction of the length L of thechannels 45 of the TFTs. The short-axis of the pulse laser line beam isparallel to the scanning direction 51 and the long axis is perpendicularto the scanning direction 51.

For example, the dimension in the scanning direction 51 (the short-axialwidth 53) of an irradiation area is several hundred micrometers and thescanning pitch 52 is several ten micrometers. The scanning pitch 52 isthe distance (the irradiation interval) by which the repeatedly emittedpulse laser beam is translated per step. The dimension of the long axisof the pulse laser line beam is usually larger than the dimension of thedisplay region in the direction parallel to this long axis.

In the example of FIG. 1E, each region of the poly-silicon film 40corresponding to one cycle of characteristics variation is separatedinto three regions 41, 42, and 43 for convenience of explanation. Withreference to the foregoing description about the crystallizationprocess, the region 41 corresponds to the region irradiated with theeven energy of the pulse laser beam and the regions 42 and 43 are theregions irradiated with the front range of the pulse laser beam.Particularly, the region 42 is meant to correspond to the regionirradiated with the range where the energy decreases; the region 43 ismeant to correspond to the region irradiated with the border range ofthe threshold for crystallization. In actual cases, however, thepoly-silicon film 40 cannot be separated by clear boundaries as shown inFIG. 1E and the characteristics of the poly-silicon film 40 changeindistinguishably from the region 41 to the region 42, and further tothe region 43.

The regions 41, 42, and 43 in this order are cyclically located insynchronization with pulse laser beam irradiation lines (the linesdetermined by the scanning pitch 52). The total dimension in thescanning direction 51 of three successive regions 41, 42, and 43 equalsto the scanning pitch 52.

It can be considered that the characteristics of the channel 45 of a TFTare substantially the same as the average of the characteristicsextracted from the poly-silicon film 40 in a range corresponding to thechannel length L. As to the example of FIG. 1E, it can be consideredthat the channels 45 composed of the same area proportions of threeregions 41, 42, and 43 have substantially the same characteristics.

A significant difference of an OLED element from a liquid crystalelement is that the OLED element emits light from itself and is drivenby electric current. Liquid crystal display devices are configured tocharge each pixel with a predetermined voltage through a pixel selectionTFT working as a switch and hold the voltage by turning off the switch(selection TFT). That is to say, the display is determined by thevoltage given from the external and held by the pixel.

OLED display devices are configured to hold a voltage given from theexternal within a pixel like liquid crystal display devices; however,the OLED display devices operate a pixel driving TFT with the voltage tocontrol the current to flow in an OLED element. If TFTs have differenttransistor characteristics, the currents to flow will be different evenwhen the TFTs are operated at the same voltage. Hence, in a pixelcircuit, transistor characteristics of the driving TFT have the largesteffect on light emission of the OLED element.

For this reason, a typical pixel circuit for an OLED display device isdesigned so that the driving TFT will operate in the saturated regionfor luminance control. Further, to avoid kink effect of a poly-siliconTFT for stable saturation characteristics, the driving TFT is designedto have a long channel. Disposing the long channel in parallel to theabove-described scanning direction of the pulse laser beam leads toaveraging the cyclic characteristics variation in the poly-silicon film.Because of this averaging, the driving TFT is less affected by thecyclic characteristics variation of the poly-silicon film 40.

Accordingly, it is important to minimize the differences incharacteristics among the channels of the driving TFTs in differentpixel circuits, particularly the pixel circuits for subpixels of thesame color.

In an aspect of this disclosure, the channels of the driving TFTsincluded in a plurality of pixel circuits for subpixels of the samecolor or all colors are disposed at substantially the same phase inirradiation cycles (spatial cycles) of a pulse laser beam in thescanning direction of the pulse laser beam. Explaining this conditionwith the coordinates of a real space, the poly-silicon film has cycliccharacteristics distribution generated by a pulse laser beam asdescribed above. Defining the irradiation point (line) of the pulselaser beam as the front end in the short-axis direction (or the scanningdirection) of the irradiation area, the poly-silicon film has theidentical patterns of characteristics distribution at regular intervals(of the scanning pitch) with reference to the irradiation lines.

Meanwhile, defining the location of the channel of a TFT as the frontend of the channel in the scanning direction of the pulse laser beam,“Disposing the channels at the same phase in irradiation cycles of apulse laser beam” means that the distances between the channels and theirradiation line closest thereto are the same among all driving TFTs.Since the channels of these driving TFTs have the identical shapes andorientations, the patterns of the characteristics of the poly-silicon ofthe channels are identical. As a result, the driving TFTs of thesubpixels of the same color can have the same characteristics.

As described above, each driving TFT is disposed in a pixel circuit tobe located at the same position with reference to an irradiation line ofthe pulse laser beam; in other words, each driving TFT is disposed atthe equivalent position (same phase) for the scanning pitch of the pulselaser beam. As a result, the display unevenness caused by laserannealing for crystallization of a silicon film can be reducedeffectively.

Disposing the channels of driving TFTs at the same phase in irradiationcycles is effective especially in the case where the channels arebending. In the case where the channels are straight in the scanningdirection and their lengths (the dimension in the scanning direction)are an integral multiple of the scanning pitch, the averagecharacteristics of the poly-silicon of the channels are substantiallythe same. In the example of FIG. 1E, when the channel length L isselected to be an integral multiple of the scanning pitch 52, the areaproportions of three regions 41, 42, and 43 constituting the channelsare the same even if the channels are located at different phases.

However, in the case of bending channels, channels disposed at differentphases in irradiation cycles can have different area proportions ofthree regions 41, 42, and 43. As described above, the channels havingdifferent area proportions of three regions 41, 42, and 43 havedifferent characteristics. Disposing the channels of driving TFTs at thesame phase in irradiation cycles in the scanning direction of the pulselaser beam as described above enables channels having a desired shape tohave the identical characteristics patterns.

In another aspect of this disclosure, the channels of the driving TFTsincluded in pixel circuits for subpixels of the same color or all colorshave a specific shape to reduce the differences in channelcharacteristics caused by locational differences among the channels.More specifically, the channels have a specific bending shape andfurther, the dimension of the channels in the scanning direction of thepulse laser beam are an integral multiple of the scanning pitch of thepulse laser beam. The channels have a specific bending shape to reducethe differences in proportions of regions located at different phases inirradiation cycles of the pulse laser beam. The details of the specificbending shape will be described later.

Because of the aforementioned specific bending shape and dimension, thechannels disposed at different locations can have small differences inchannel characteristics. These channels do not need to be disposed atthe same phase in irradiation cycles of the pulse laser beam.

These channels can be disposed at the same phase in irradiation cyclesof the pulse laser beam. As a result, the differences in channelcharacteristics among the channels disposed at different locations canbe further reduced. Alternatively, channels having the aforementionedspecific bending shape but having a dimension different from an integralmultiple of the scanning pitch in the scanning direction of the pulselaser beam may be disposed at the same phase in irradiation cycles ofthe pulse laser beam. As a result, the differences in channelcharacteristics caused by fluctuations in location of the channels canbe reduced.

Hereinafter, embodiments of this disclosure will be specificallydescribed with reference to the accompanying drawings. Elements commonto the drawings are denoted by the same reference signs. For clearunderstanding of the description, the elements in the drawings may beexaggerated in size or shape.

Embodiment 1 Overall Configuration

FIG. 2 schematically illustrates a configuration example of an OLEDdisplay device 10. The OLED display device 10 includes a thin filmtransistor (TFT) substrate 100 on which OLED elements are formed, anencapsulation substrate 200 for encapsulating the OLED elements, and abond (glass frit sealer) 300 for bonding the TFT substrate 100 with theencapsulation substrate 200. The space between the TFT substrate 100 andthe encapsulation substrate 200 is filled with dry air and sealed upwith the bond 300.

In the periphery of a cathode electrode forming region 114 outer thanthe display region 125 of the TFT substrate 100, a scanning driver 131,an emission driver 132, a protection circuit 133, a driver IC 134, and ademultiplexer 136 are provided. The driver IC 134 is connected to theexternal devices via flexible printed circuits (FPC) 135.

The scanning driver 131 drives scanning lines on the TFT substrate 100.The emission driver 132 drives emission control lines to control thelight emission periods of pixels. The protection circuit 133 protectsthe elements from electrostatic discharge. The driver IC 134 is mountedwith an anisotropic conductive film (ACF), for example.

The driver IC 134 provides power and timing signals (control signals) tothe scanning driver 131 and the emission driver 132 and further,provides power and a data signal to the demultiplexer 136.

The demultiplexer 136 outputs output of one pin of the driver IC 134 tod data lines in series (d is an integer more than 1). The demultiplexer136 changes the output data line for the data signal from the driver IC134 d times per scanning period to drive d times as many data lines asoutput pins of the driver IC 134.

Circuit Configuration

A plurality of pixel circuits are formed on the substrate 100 to controlelectric current to be supplied to the anode electrodes of subpixels.FIG. 3A illustrates a configuration example of a pixel circuit. Eachpixel circuit includes a driving transistor T1, a selection transistorT2, an emission transistor T3, and a storage capacitor C1. The pixelcircuit controls light emission of an OLED element El. The transistorsare TFTs.

The selection transistor T2 is a switch for selecting the sub-pixel. Theselection transistor T2 is a p-channel TFT and its gate terminal isconnected with a scanning line 106. The source terminal is connectedwith a data line 105. The drain terminal is connected with the gateterminal of the driving transistor T1.

The driving transistor T1 is a transistor (driving TFT) for driving theOLED element E1. The driving transistor T1 is a p-channel TFT and itsgate terminal is connected with the drain terminal of the selectiontransistor T2. The source terminal of the driving transistor T1 isconnected with a power line (Vdd) 108. The drain terminal is connectedwith the source terminal of the emission transistor T3. The storagecapacitor C1 is provided between the gate terminal and the sourceterminal of the driving transistor T1.

The emission transistor T3 is a switch for controlling supply/stop ofthe driving current to the OLED element E1. The emission transistor T3is a p-channel TFT and its gate terminal is connected with an emissioncontrol line 107. The source terminal of the emission transistor T3 isconnected with the drain terminal of the driving transistor T1. Thedrain terminal of the emission transistor T3 is connected with the OLEDelement E1.

Next, operation of the pixel circuit is described. The scanning driver131 outputs a selection pulse to the scanning line 106 to turn on thetransistor T2. The data voltage supplied from the driver IC 134 throughthe data line 105 is stored to the storage capacitor C1. The storagecapacitor C1 holds the stored voltage during the period of one frame.The conductance of the driving transistor T1 changes in an analog mannerin accordance with the stored voltage, so that the driving transistor T1supplies a forward bias current corresponding to a light emission levelto the OLED element E1.

The emission transistor T3 is located on the supply path of the drivingcurrent. The emission driver 132 outputs a control signal to theemission control line 107 to control ON/OFF of the emission transistorT3. When the emission transistor T3 is ON, the driving current issupplied to the OLED element E1. When the emission transistor T3 is OFF,this supply is stopped. The lighting period (duty ratio) in the periodof one field can be controlled by controlling ON/OFF of the transistorT3.

FIG. 3B illustrates another configuration example of a pixel circuit.This pixel circuit includes a reset transistor T4 in place of theemission transistor T3 in FIG. 3A. The reset transistor T4 controls theelectric connection between a reference voltage supply line 110 and theanode of the OLED element E1. This control is performed in accordancewith a reset control signal supplied from a reset control line 109 tothe gate of the reset transistor T4.

The reset transistor T4 can be used for various purposes. For example,the reset transistor T4 can be used to reset the anode electrode of theOLED element E1 once to a sufficiently low voltage that is lower thanthe black signal level to prevent crosstalk caused by leak currentbetween OLED elements E1.

The reset transistor T4 can also be used to measure a characteristic ofthe driving transistor T1. For example, the voltage-currentcharacteristic of the driving transistor T1 can be accurately measuredby measuring the current flowing from the power line (Vdd) 108 to thereference voltage supply line (Vref) 110 under the bias conditionsselected so that the driving transistor T1 will operate in the saturatedregion and the reset transistor T4 will operate in the linear region. Ifthe differences in voltage-current characteristic among the drivingtransistors T1 for individual subpixels are compensated for bygenerating data signals at an external circuit, a highly-uniform displayimage can be attained.

In the meanwhile, the voltage-current characteristic of the OLED elementE1 can be accurately measured by applying a voltage to light the OLEDelement E1 from the reference voltage supply line 110 when the drivingtransistor T1 is off and the reset transistor T4 is operating in thelinear region. In the case where the OLED element E1 is deterioratedbecause of long-term use, for example, if the deterioration iscompensated for by generating a data signal at an external circuit, thedisplay device can have a long life spun.

The circuit configurations in FIGS. 3A and 3B are examples; the pixelcircuit may have a different circuit configuration. Although the pixelcircuits in FIGS. 3A and 3B include p-channel TFTs, the pixel circuitmay employ n-channel TFTs. The above-described pixel circuit is providedto compensate for variations in threshold voltage among the drivingtransistors to prevent impairment of the image quality. The technicalmeans described in this specification to eliminate the characteristicsdifferences among the transistors reduces the display unevenness thatcannot be sufficiently reduced by the pixel circuits.

Pixel Structure

Next, general structures of a pixel circuit and a light-emitting elementare described. FIG. 4 schematically illustrates a cross-sectionalstructure of a part of an OLED display device 10 that includes drivingTFTs. The OLED display device 10 includes a TFT substrate 100 and anencapsulation substrate 200 opposed to the TFT substrate 100. In thefollowing description, the definitions of top and bottom correspond tothe top and the bottom of the drawing.

The OLED display device 10 includes an insulating substrate 151 and anencapsulation structural unit opposed to the insulating substrate 151.An example of the encapsulation structural unit is a flexible orinflexible encapsulation substrate 200. The encapsulation structuralunit can be a thin film encapsulation (TFE) structure.

The OLED display device 10 includes lower electrodes (for example, anodeelectrodes 162), upper electrodes (for example, cathode electrodes 166),and organic light-emitting films 165 disposed between the insulatingsubstrate 151 and the encapsulation structural unit.

The organic light-emitting films 165 are provided between the cathodeelectrodes 166 and the anode electrodes 162. The plurality of anodeelectrodes 162 are disposed on the same plane (for example, on aplanarization film 161) and an organic light-emitting film 165 isdisposed on an anode electrode 162. In the example of FIG. 4, thecathode electrode 166 of one subpixel is a part of an unseparatedconductor film.

The OLED display device 10 further includes a plurality of post spacers(PS) 164 standing toward the encapsulation structural unit and aplurality of pixel circuits each including a plurality of switches. Eachof the plurality of pixel circuits is formed between the insulatingsubstrate 151 and an anode electrode 162 and controls the electriccurrent to be supplied to the anode electrode 162.

FIG. 4 illustrates an example of a top-emission pixel structure, whichincludes top-emission type of OLED elements. The top-emission pixelstructure is configured in such a manner that a cathode electrode 166common to a plurality of pixels is provided on the light emission side(the upper side of the drawing). The cathode electrode 166 has a shapethat fully covers the entire display region 125. The top-emission pixelstructure is characterized by that the anode electrodes 162 have lightreflectivity and the cathode electrode 166 has light transmissivity.Hence, a configuration to transmit light coming from the organiclight-emitting films 165 toward the encapsulation structural unit isattained.

Compared to a bottom-emission pixel structure configured to extractlight from the insulating substrate 151, the top-emission type does notneed a light transmissive region within a pixel region to extract light.For this reason, the top-emission type has high flexibility in layingout pixel circuits. For example, the light-emitting unit can be providedabove the pixel circuits or lines.

The top-emission pixel structure easily allows driving TFTs (thechannels thereof) to be disposed at desirable locations suitable forirradiation with a pulse laser beam for silicon laser annealing. Thebottom-emission pixel structure has a transparent anode electrode and areflective cathode electrode to transmit light to the external throughthe insulating substrate 151. The TFT layout of this disclosure is alsoapplicable to the bottom-emission pixel structure.

A subpixel of a full-color OLED display device usually displays one ofthe colors of red, green, and blue. A red subpixel, a green subpixel,and a blue subpixel constitute one main pixel. A pixel circuit includinga plurality of thin film transistors controls light emission of an OLEDelement associated therewith. An OLED element is composed of an anodeelectrode of a lower electrode, an organic light-emitting film, and acathode electrode of an upper electrode.

The insulating substrate 151 is made of glass or resin, for example, andis flexible or inflexible. A poly-silicon layer is provided above theinsulating substrate 151 with a first insulating film 152 interposedtherebetween. The poly-silicon layer includes channels 155 at thelocations where gate electrodes 157 are to be formed later. Each channel155 determines the transistor characteristics of the TFT. At both endsof each channel 155, a source region 168 and a drain region 169 areprovided. The source region 168 and the drain region 169 are doped withhigh-concentration impurities for electrical connection with a wiringlayer thereabove.

Lightly doped drains (LDDs) doped with low-concentration impurities canbe provided between the channel 155 and the source region 168 andbetween the channel 155 and the drain region 169. FIG. 4 omits the LDDsto avoid complexity. Above the poly-silicon layer, gate electrodes 157are provided with a gate insulating film 156 interposed therebetween. Aninterlayer insulating film 158 is provided above the layer of the gateelectrodes 157.

Within the display region 125, source electrodes 159 and drainelectrodes 160 are provided above the interlayer insulating film 158.The source electrodes 159 and the drain electrodes 160 are formed of ametal having a high melting point or an alloy of such a metal. Eachsource electrode 159 and each drain electrode 160 are connected with asource region 168 and a drain region 169 of the poly-silicon layerthrough contact holes 170 and 171 provided in the interlayer insulatingfilm 158 and the gate insulating film 156.

Over the source electrodes 159 and the drain electrodes 160, aninsulative planarization film 161 is provided. Above the insulativeplanarization film 161, anode electrodes 162 are provided. Each anodeelectrode 162 is connected with a drain electrode 160 through a contactprovided in a contact hole 172 in the planarization film 161. The TFTsof a pixel circuit are formed below the anode electrode 162.

Above the anode electrodes 162, an insulative pixel defining layer (PDL)163 is provided to separate OLED elements. OLED elements are formed inopenings 167 of the pixel defining layer 163. Insulative spacers 164 areprovided on the pixel defining layer 163 to be located between anodeelectrodes 162 and maintain the space between the OLED elements and theencapsulation substrate 200.

Above each anode electrode 162, an organic light-emitting film 165 isprovided. The organic light-emitting film 165 is in contact with thepixel defining layer 163 in the opening 167 of the pixel defining layer163 and its periphery. A cathode electrode 166 is provided over theorganic light-emitting film 165. The cathode electrode 166 is alight-transmissive electrode. The cathode electrode 166 transmits all orpart of the visible light coming from the organic light-emitting film165. The laminated film of the anode electrode 162, the organiclight-emitting film 165, and the cathode electrode 166 formed in anopening 167 of the pixel defining layer 165 corresponds to an OLEDelement. A not-shown cap layer may be provided over the cathodeelectrode 166.

Manufacturing Method

An example of the method of manufacturing the OLED display device 10 isdescribed. The method of manufacturing the OLED display device 10 firstdeposits silicon nitride, for example, onto the insulating substrate 151by chemical vapor deposition (CVD) to form a first insulating film 152.Next, the method forms a layer (poly-silicon layer) including channels155 by a known low-temperature poly-silicon TFT fabrication technique.Specifically, the method forms the poly-silicon layer by depositingamorphous silicon by CVD and crystalizing the amorphous silicon byexcimer laser annealing (ELA) described with reference to FIG. 1A. Themethod processes the poly-silicon film to have island-like shapes anddopes the source and drain regions 168 and 169 to be connected withsource electrodes 159 and drain electrodes 160 with impurities in highconcentration to reduce the resistance. The poly-silicon layer reducedin resistance can also be used to connect elements within the displayregion 125.

Next, the method deposits silicon oxide, for example, onto thepoly-silicon layer including the channels 155 by CVD to form a gateinsulating film 156. Furthermore, the method deposits a metal bysputtering and patterns the metal to form a metal layer including gateelectrodes 157.

The metal layer includes storage capacitor electrodes, scanning lines106, and emission control lines, in addition to the gate electrodes 157.The metal layer may be a single layer made of one material selected froma group consisting of Mo, W, Nb, MoW, MoNb, Al, Nd, Ti, Cu, a Cu alloy,an Al alloy, Ag, and an Ag alloy. Alternatively, the metal layer may bea laminated layer to reduce the wiring resistance. The laminated layerhas a multi-layer structure including two or more layers each made of alow-resistive material such as Mo, Cu, Al, Ag, or an alloy thereof.

In forming the metal layer, the method keeps offset regions to the gateelectrodes 157 in the source and drain regions 168 and 169.Subsequently, the method dopes the poly-silicon film with additionalimpurities using the gate electrodes 157 as a mask to provide a layer oflow-concentration impurities between the source regions 169 and thechannels 155 located under the gate electrodes 157 and between the drainregions 168 and the channels 155. As a result, the TFTs has a lightlydoped drain (LDD) structure. Next, the method deposits silicon oxide byCVD to form an interlayer insulating film 158.

The method opens contact holes in the interlayer insulating film 158 andthe gate insulating film 156 by anisotropic etching. The contact holes170 and 171 to connect the source electrodes 159 and the drainelectrodes 160 to the source regions 168 and the drain regions 169,respectively, are formed in the interlayer insulating film 158 and thegate insulating film 156.

Next, the method deposits conductive materials such as Ti/Al/Ti bysputtering and patterns the film to form a metal layer. The metal layerincludes source electrodes 159, drain electrodes 160, and inner walls ofthe contact holes 170 and 171. In addition to these, data lines 105 andpower lines 108 are also formed on the same layer.

Next, the method deposits a photosensitive organic material to form aplanarization film 161. Subsequently, the method opens contact holes 172connecting to the source electrodes 159 and the drain electrodes 160 ofthe TFTs by exposure and development. The method forms anode electrodes162 on the planarization film 161 having contact holes 172. An anodeelectrode 162 includes three layers of a transparent film made of ITO,IZO, ZnO, In₂O₃, or the like, a reflective film made of a metal such asAg, Mg, Al, Pt, Pd, Au, Ni, Nd, Ir, or Cr or an alloy containing such ametal, and another transparent film as mentioned above. The three-layerstructure of the anode electrode 162 is merely an example and the anodeelectrode 162 may have a two-layer structure. The anode electrodes 162are connected to the drain electrodes 160 through the contact holes 172.

Next, the method deposits a photosensitive organic resin by spin coatingand patterns the photosensitive organic resin to form a pixel defininglayer 163. The patterning creates openings 167 in the pixel defininglayer 163 to expose the anode electrodes 162 of the sub-pixels at thebottom of the created openings 167. The inner walls of the openings 167in the pixel defining layer 163 are normally tapered. The pixel defininglayer 163 forms separate light-emitting regions of sub-pixels. Themethod further deposits a photosensitive organic resin by spin coatingand patterns the photosensitive organic resin to form spacers 164 on thepixel defining layer 163.

Next, the method applies organic light-emitting materials onto theinsulating substrate 151 provided with the pixel defining layer 163 toform organic light-emitting films 165. An organic light-emitting film165 is formed by depositing an organic light-emitting material for thecolor of R, G, or B on an anode electrode 162. The forming an organiclight-emitting film 165 uses a metal mask. The organic light-emittingfilm 165 consists of, for example, a hole injection layer, a holetransport layer, a light-emitting layer, an electron transport layer,and an electron injection layer in this order from the bottom. Thelaminate structure of the organic light-emitting film 165 is determineddepending on the design.

Next, the method applies a metal material for the cathode electrode 166onto the TFT substrate 100 where the pixel defining layer 163, thespacers 164, and the organic light-emitting films 165 (in the openingsof the pixel defining layer 163) are exposed. The metal materialdeposited on the organic light-emitting film 165 of one sub-pixelfunctions as the cathode electrode 166 of the sub-pixel within theregion of an opening of the pixel defining layer 163.

The layer of the cathode electrode 166 is formed by vapor-deposition ofa metal such as Al or Mg or an alloy thereof, for example. If theresistance of the cathode electrode 166 is so high to impair theuniformity of the luminance of the emitted light, an additionalauxiliary electrode layer may be formed using a material for atransparent electrode, such as ITO, IZO, ZnO, or In₂O₃.

Layout of Driving Transistors

Hereinafter, examples of the layout of driving transistors T1 in aplurality of pixel circuits within a display region are described.Particularly, relations between the locations of the channels of thedriving TFTs and the irradiation points (lines) of the ELA pulse laserbeam for preparing poly-silicon are described in detail. For simplicityof explanation, examples employing pixels having the pixel circuitconfiguration illustrated in FIG. 5 are described. The pixel circuit inFIG. 5 has a configuration in which the emission transistor T3 and theemission control line 107 are omitted from the pixel circuit illustratedin FIG. 3A. The following description is applicable to other pixelcircuit configurations like those illustrated in FIGS. 3A and 3B.

FIG. 6 is a plan diagram of an example of the layout of the elements ofa pixel circuit having the circuit configuration illustrated in FIG. 5.The pixel circuit includes a driving transistor T1, a selectiontransistor T2, and a storage capacitor C1 for the driving transistor T1.The active layers of the TFTs are made from a poly-silicon film. Theoverlap region of the poly-silicon film with a gate electrode 157 is achannel 155.

The ends of the channel 155 are continued to a source region 168 and adrain region 169 doped with high-concentration impurities (in somecases, via an LDD region doped with low-concentration impurities) forelectrical connection with a source electrode 159 and a drain electrode160, respectively. A scanning line 106 and the lower electrode of thestorage capacitor C1 are formed on the same metal layer (lower metallayer) as the gate electrode 157. A data line 105, a power line 108, andthe upper electrode of the storage capacitor C1 are formed on the samemetal layer (upper metal layer) as the source electrode 159 and thedrain electrode 160.

The source electrode 159 of the driving transistor T1 interconnects thepower line 108 and the source region 168 of the poly-silicon layerthrough a contact hole 170. The drain electrode 160 interconnects theanode electrode 162 (not shown in FIG. 5) and the drain region 169 ofthe poly-silicon layer through a contact hole 171.

The channel 155 is a part of the poly-silicon film of the drivingtransistor T1 covered by the gate electrode 157 when seen from the top.In the example of FIG. 6, the channel 155 is straight and extends inparallel to the direction (the vertical direction in FIG. 6) in whichthe data line 105 or the power line 108 extends.

In the example of FIG. 6, the channel 155 of the driving transistor T1has a channel length of 40 μm and a channel width of 4 μm. The pixelpitch P_(PIX) in the direction (the vertical direction in FIG. 6) inwhich the data line 105 or the power line 108 extends is 103.5 μm. Aswill be described later, the scanning direction of the ELA system with apulse laser beam is parallel to this pixel pitch. The layout and thenumerical values of the pixel elements provided in FIG. 6 are merelyexamples; the features of this disclosure are applicable to pixelshaving different configurations and/or sizes.

In the following description, the channels 155 of the drivingtransistors T1 of all subpixels are designed to have the identicalshapes (inclusive of sizes) and orientations and therefore, have thesame channel length and channel width. In another example, the channels155 of the driving transistors T1 may have different shapes depending onthe color of the subpixels. The shapes, orientations, lengths, andwidths of the channels 155 could be slightly different among individualsubpixels because of the irregularity in manufacture.

FIG. 7 schematically illustrates a positional relation among pixels 140disposed in the scanning direction 51 of a pulse laser beam 50, thechannels 155 of the driving transistors T1 in the pixels, and successiveirradiation points (lines) 56 of the pulse laser beam 50. The rectangles1, 2, . . . n of the pixels 140 schematically represent the areasoccupied by the individual pixels 140 disposed at a pixel pitch P_(PIX).For example, each rectangle is defined by connecting the boundaries atthe middle between light-emitting regions adjacent to each other. Notethat FIGS. 7 to 17 and the description thereof simply refer to subpixelas pixel unless otherwise specified.

An irradiation line 56 is a specific position in the short-axisdirection, for example the front end in the scanning direction, of anarea irradiated with a shot of pulse laser beam. The irradiation line 56can be any position that can define the scanning pitch P_(ELA) of thepulse laser beam 50.

The channels 155 of the driving transistors T1 included in the pixelcircuits of the individual pixels 140 are disposed at the same phase inirradiation cycles (locational cycles) of the pulse laser beam when seenin the scanning direction 51 of the pulse laser beam. In the example ofFIG. 7, the upper end of each channel 155 coincides with an irradiationline 56. The distance between the channels of two successive pixels isan integral multiple of the scanning pitch P_(ELA).

Disposing the channels 155 to be located at different positions in thescanning direction 51 at the same phase in irradiation cycles leads thepoly-silicon of the channels 155 to have identical characteristicsvariation patterns, enabling the driving transistors T1 to have the samecharacteristics.

As described above, each channel 155 is disposed at the same positionwith reference to an irradiation line of a pulse laser beam, in otherwords, each channel 155 is disposed at an equivalent position for thescanning pitch of a pulse laser beam. As a result, display unevennessthat could occur because of leaser annealing to prepare a poly-siliconfilm can be reduced effectively.

The scanning pitch is usually smaller than the pixel pitch. In the casewhere driving TFTs of different pixel circuits are disposed at the samerelative position to the areas occupied by the corresponding pixels, thedriving TFTs of pixels adjacent to each other in the scanning directionare located at different positions with respect to the scanning pitch bythe residue obtained by dividing the pixel pitch by the scanning pitch.The positions of the channels 155 are determined to coincide with cyclicirradiation lines 56 and this condition needs to be satisfied in theentire display region. If a pixel unit consisting of some pixels isdefined and configured to satisfy the foregoing condition within thepixel unit, the layout can be easily expanded to the entire displayregion.

In FIG. 7, successive n pixels 140 are disposed in a line in thescanning direction 51 of the pulse laser beam 50, where n is a naturalnumber. The n pixels 140 constitute one pixel unit; the size of thepixel unit in the scanning direction 51 is P_(UNIT). A relationP_(UNIT)=n×P_(PIX)=N×P_(ELA) is satisfied, where N is a natural numberand represents the number of shots of a pulse laser beam in a P_(UNIT)(hereinafter, referred to as the number of ELA cycles). In the exampleof FIG. 7, the scanning pitch P_(ELA) is smaller than the pixel pitchP_(PIX) and N is greater than n.

A plurality of pixel units are disposed in a matrix to be a displayregion 125. Accordingly, the positional pattern of the channels of thedriving transistors T1 in a pixel unit is repeated in the scanningdirection. In addition, the relation between the pixels 140 included ina pixel unit and the irradiation lines 56 is also repeated in thescanning direction because the relation of P_(UNIT)=n×P_(PIX)=N×P_(ELA)is satisfied. As understood from the foregoing description, the pixelcircuit layout of the entire display region 125 can be designedefficiently by determining a pixel circuit layout in a pixel unit.

The scanning pitch P_(ELA) needs to be selectable on the ELA system. Acommon ELA system is configured to allow setting of the scanning pitchP_(ELA) with an integer in unit of μm. Even in the case of another ELAsystem that accepts a number with a decimal point, selecting an integeror a simple value that does not uselessly increase the number of digitsafter the decimal point for the scanning pitch P_(ELA) reduces theprobability of erroneous operation because of hardware accuracy.

In designing and manufacturing an OLED display device 10, the values forN, n, and P_(ELA) are determined to satisfy the relation ofN×P_(ELA)=n×P_(PIX) for a predetermined pixel pitch P_(PIX). Afterdetermination of these values, the positions of the driving transistorsT1 (the channels 155 thereof) in individual pixels 140 in a pixel unitare determined to be the same phase in irradiation cycles.

In this example, the pixel pitch P_(PIX) is 103.5 μm. FIG. 8 providesexamples of the combination of N, n, and P_(ELA) that satisfy therelation of N×P_(ELA)=n×P_(PIX). A desired integer for P_(ELA) can beefficiently obtained by calculating n×P_(PIX)=P_(UNIT) with differentvalues of n and factorizing the values of P_(UNIT) into prime factors,for example.

For example, when the number of pixels per pixel unit n is 2,P_(UNIT)=n×P_(PIX) is 207. This value is prime-factorized as 3²×23.Accordingly, the available values for P_(ELA) are 3, 9, 23, 69, and 207.If the preferable P_(ELA) is about 20 μm, 23 μm is selected as P_(ELA).The number of ELA cycles N in this case is 9. In similar, when thenumber of pixels n is 4, the value closest to 20 μm is 18 μm; when thenumber of pixels n is 14, the value closest to 20 μm is 21 μm. Thenumber of ELA cycles N in these cases are 23 and 69.

Hereinafter, three layout examples in accordance with the foregoingnumerical values are described. FIG. 9 provides a layout in theconditions where the pixel pitch P_(PIX) is 103.5 μm, the scanning pitchP_(ELA) is 18 μm, the number of pixels per pixel unit n is 4, and thenumber of ELA cycles N is 23. FIG. 10 provides numerical values forspecifying this layout.

A pixel unit consists of four pixels 140A to 140D. The distance(TFT-to-TFT distance) between the channels 155 of the drivingtransistors T1 of the pixels 140A and 140B is six times of the scanningpitch P_(ELA) (6×P_(ELA)). The distance between the channels 155 of thedriving transistors T1 of the pixels 140B and 140C is six times of thescanning pitch P_(ELA). The distance between the channels 155 of thedriving transistors T1 of the pixels 140C and 140D is six times of thescanning pitch P_(ELA). The distance between the channels 155 of thedriving transistors T1 of the pixel 140D and the pixel 140A in the nextpixel unit is five times of the scanning pitch P_(ELA) (5×P_(ELA)).

The accumulated distance in FIG. 10 indicates the total sum of the pixelpitches P_(PIX) from the pixel 140A to the pixel of the entry. Therelative position in FIG. 10 indicates the relative position of thechannel 155 (the driving transistor T1) in the pixel circuit for thepixel of the entry. As illustrated in FIG. 11, the position in the pixelcircuit of the channel 155 in the pixel 140A is defined as referenceposition. The number of ELA cycles and the TFT-to-TFT distance in FIG.10 are the values from the pixel of the entry to the next pixel in thescanning direction. Note that FIG. 11 does not depict an actual pixelarray composed of pixels disposed side by side but depicts a state whereindividual pixels disposed one above another in the pixel arrayillustrated in FIG. 9 are disposed side by side.

The relative position of the channel 155 in the pixel 140B is differentfrom the reference position by 4.5 μm in the scanning direction 51. Therelative positions of the channels 155 in the pixels 140C and 140D aredifferent from the reference position by 9.0 μm and 13.5 μm,respectively, in the scanning direction 51.

FIG. 12 provides a layout in the conditions where the pixel pitchP_(PIX) is 103.5 μm, the scanning pitch P_(ELA) is 23 μm, the number ofpixels per pixel unit n is 2, and the number of ELA cycles N is 9. FIG.13 provides numerical values for specifying this layout.

A pixel unit consists of two pixels 240A and 240B. The distance(TFT-to-TFT distance) between the channels 155 of the drivingtransistors T1 of the pixels 240A and 240B is five times of the scanningpitch P_(ELA). The distance between the channels 155 of the drivingtransistors T1 of the pixel 240B and the pixel 240A in the next pixelunit is four times of the scanning pitch P_(ELA). The distance betweenthe channels of pixels adjacent to each other is an integral multiple ofthe scanning pitch P_(ELA) and the largest difference among thechannel-to-channel distances is one scanning pitch P_(ELA).

The accumulated distance in FIG. 13 indicates the total sum of the pixelpitches P_(PIX) from the pixel 240A to the pixel of the entry. Therelative position in FIG. 13 indicates the relative position of thechannel 155 (the driving transistor T1) in the pixel circuit for thepixel of the entry. As illustrated in FIG. 14, the position in the pixelcircuit of the channel 155 in the pixel 240A is defined as referenceposition. The number of ELA cycles and the TFT-to-TFT distance in FIG.13 are the values from the pixel of the entry to the next pixel in thescanning direction. The relative position of the channel 155 in thepixel 240B is different from the reference position by 11.5 μm in thescanning direction 51. Note that FIG. 14 does not depict an actual pixelarray composed of pixels disposed side by side but depicts a state whereindividual pixels disposed one above the other in the pixel arrayillustrated in FIG. 12 are disposed side by side.

FIG. 15 provides a layout in the conditions where the pixel pitch PpIxis 103.5 μm, the scanning pitch P_(ELA) is 21 μm, the number of pixelsper pixel unit n is 14, and the number of ELA cycles N is 69. FIG. 16provides numerical values for specifying this layout.

A pixel unit consists of fourteen pixels 340A to 340N. The distances(TFT-to-TFT distance) between the channels 155 of the drivingtransistors T1 of pixels adjacent to each other are four times of thescanning pitch P_(ELA) only between the pixel 340N and the pixel 340A ofthe next pixel unit and five times of the scanning pitch P_(ELA) betweenthe other pixels.

The accumulated distance in FIG. 16 indicates the total sum of the pixelpitches P_(PIX) from the pixel 340A to the pixel of the entry. Therelative position in FIG. 16 indicates the relative position of thechannel 155 (the driving transistor T1) in the pixel circuit for thepixel of the entry. As illustrated in FIG. 17, the position in the pixelcircuit of the channel 155 in the pixel 340A is defined as referenceposition. The relative positions of the channels 155 in the pixels 340Bto 340N are provided in FIG. 16. FIG. 17 shows the relative positions ofthe channels 155 in the pixels 340A and 340N selected from the pixels340A to 340N. The number of ELA cycles and the TFT-to-TFT distance inFIG. 16 are the values from the pixel of the entry to the next pixel inthe scanning direction.

Positioning the driving transistors T1 within the pixel circuits asdescribed above leads the channels 155 in different pixel circuits to belocated at the same phase with respect to irradiation lines. Eachchannel-to-channel distance between pixels adjacent to each other is anintegral multiple of the scanning pitch P_(ELA) and in the foregoingdescription, the largest difference among the channel-to-channeldistances is one scanning pitch P_(ELA). Such a small difference inchannel-to-channel distance facilitates designing a pixel circuitstructure.

It should be noted the functions and effects of this invention areattained as far as each channel-to-channel distance between adjacentpixels is an integral multiple of the scanning pitch P_(ELA). Even whenthe largest difference among the channel-to-channel distances is largerthan one scanning pitch P_(ELA), the same effects can be attained ifdesigning a pixel circuit structure is available.

The foregoing description is based on FIGS. 10, 13, and 16 in which thepixels are listed in such a manner that the relative positions take avalue of 0 or a positive value. If the pixel to be the reference ischanged in the same pixel disposition, one or more relative positionswill take a negative value. However, note that the pixel disposition isunchanged.

When channels are located at the same phase in ELA irradiation cycles,the channels can have the same characteristics, independently from theshape of the channels. In actual manufacture, however, channels could beslightly dislocated from the designed positions. As far as the channelsare straight and their sizes in the scanning direction are an integralmultiple of the scanning pitch, the characteristics differences amongthe channels can be made small even if the channels are slightlydislocated from the designed positions.

In the above-described examples, each pixel circuit is disposed withinthe area of the subpixel to be driven by the pixel circuit when seenfrom the top. However, the area occupied by a pixel circuit does notneed to be included in the area of the light-emitting region of theassociated subpixel when seen from the top. Especially, top emissiontype of display devices have high flexibility in layout of pixelcircuits and the light-emitting regions of subpixels; the area of apixel circuit of a pixel may overlap partially or wholly with the areaof the light-emitting region of an adjacent subpixel. The area includingthe light-emitting region of a subpixel may include the pixel circuit ofanother subpixel. Subpixels of different colors can have pixel circuitshaving different element layouts. Subpixels of the same color can havepixel circuits having different element layouts. For example, a displaydevice may have pixel circuits structured to be symmetric about avirtual axis.

The channels of the driving TFTs of all pixels in the display region 125do not need to be located at the same phase in ELA irradiation cycles.For example, the channels of the driving TFTs of pixels of differentcolors can be located at different phases in ELA irradiation cycles asfar as the channels of the driving TFTs of pixels of the same color arelocated at the same phase in ELA irradiation cycles.

The above-described layout of the channels of driving TFTs is applicableto transistors other than the driving TFTs or transistors in a displaydevice of a type other than the OLED display device. In addition, thislayout is applicable to transistors in a device including TFTs arrayedin a matrix. Furthermore, this layout is applicable to transistors of akind other than poly-silicon transistors that are to be annealed with apulse laser beam. Such transistors include oxide semiconductortransistors to be annealed with a pulse laser beam.

Embodiment 2 Bending Channels

Hereinafter, driving TFTs having bending channels are described.Differences from Embodiment 1 are mainly described in the following. Thedescription provided in Embodiment 1 is applicable to Embodiment 2,except for the description about the shapes and the positions of thechannels.

The trend of display devices for higher resolution has encouragedemployment of bending channels so as to dispose driving TFTs having along channel length L within a small area. FIG. 18 schematicallyillustrates an example of the positional relation of a bending channel551 to irradiation lines 56 of a pulse laser beam. For convenience ofreferencing, the rectangle in dashed lines surrounding a channel isdenoted by a reference sign 551. This way of referencing the channel isused in the subsequent drawings.

The bending channel 551 is formed in such a manner that first sectionsextending in the scanning direction 51 and second sections extending inthe direction perpendicular to the scanning direction 51 are alternatelyconnected. The channel 551 has a channel length L and a channel width W.

As will be described later, in the case where channels are bending andlocated at different phases in irradiation cycles, the proportion of thechannel regions having different characteristics can be different amongthe channels, even if the channel size LB in the scanning direction 51is an integral multiple of the scanning pitch P_(ELA). For this reason,the individual driving TFTs could show different characteristics.

Hereinafter, some examples of the shape of a bending channel aredescribed. FIG. 19 illustrates a bending channel 551A and a distribution559A of areas occupied by the channel 551A at each position in thescanning direction 51. The channel 551A has a channel length L, achannel width W, and a channel size LB1 in the scanning direction 51.

The distribution 559A provides the sum of the lengths in the directionperpendicular to the scanning direction 51 of the channel 551A (the areaoccupied by the channel 551A) at each position in the scanning direction51. In other words, defining virtual lines extending perpendicularly tothe scanning direction 51 at different positions in the scanningdirection 51, the distribution 559A provides the sum of the lengths ofthe overlap segments of each virtual line with the channel 551A.

As noted from the shape of the distribution 559A, the channel 551A inFIG. 19 has large differences in total length (or area) among thepositions in the scanning direction 51. The distribution 559A in FIG. 19has two peaks 560 at positions including irradiation points (lines) 56.Another channel located at a different phase of an ELA irradiation cyclehas these peaks 560 at different positions. That is to say, theproportions of the regions having different characteristics aresignificantly different among the channels of different subpixels. Asdescribed above, when different channels are located at the same phasein ELA irradiation cycles, they show the same channel characteristics.When those channels are located at different phases, however, theirchannel characteristics could be significantly different.

FIG. 20 illustrates another bending channel 551B and a distribution 559Bof areas occupied by the channel 551B at each position in the scanningdirection 51. The channel 551B has a channel length L, a channel widthW, and a channel size LB2 in the scanning direction 51.

As indicated in the distribution 559B, the channel 551B has a uniformtotal length (or area) at each position in the scanning direction 51. Inother words, the total length at every position in the scanningdirection 51 is the same value. Accordingly, if the channel size LB isan integral multiple of the scanning pitch P_(ELA), the differences inproportion of the channel regions having different characteristics amongTFTs become small or substantially eliminated.

The foregoing description about FIG. 20 is provided assuming that thecharacteristics of the transistors having a bending channel are thesynthetic characteristics of the channel portions at individual phases,independently from the direction of the electric current. The reason whysuch a description is viable will be described later.

FIG. 21 is a diagram for illustrating the details of the shape of thechannel 551B in FIG. 20. The channel 551B has a shape such that aplurality of rectangles are combined; its straight parts along theelectric current path have a uniform channel width W. The channel lengthL (see FIG. 20) is defined along the central line of the electriccurrent path between the channel ends 556 and 557. The channel 551Bconsists of sections 553A, 553B, and 553C extending in the firstdirection and sections 554A and 554B extending in the second direction.The first direction is parallel to the scanning direction 51 and thesecond direction is perpendicular to the scanning direction 51.

In FIG. 21, the scanning direction 51 is directed from the top to thebottom of the drawing. In the subsequent FIGS. 21 to 24, the firstdirection is denoted by 51, which is the same as the reference sign ofthe scanning direction. The second direction is directed from the rightto the left or from the left to the right of the drawing. In thefollowing description, the second direction is defined as the direction54 directed from the left to the right of the drawing.

In the channel 551B, the sections extending in the first direction andthe sections extending in the second direction are connectedalternately. Specifically, the section 553A extending in the firstdirection, the section 554A extending in the second direction, thesection 553B extending in the first direction, the section 554Bextending in the second direction, and the section 553C extending in thefirst direction are connected in this order.

Regarding each section extending in the second direction, at least oneend thereof is connected with a side extending along the first directionof an end part in the first direction of a section extending in thefirst direction. Specifically, the section 554A extending in the seconddirection is connected with the sections 553A and 553B extending in thefirst direction at both ends.

The left end of the section 554A is connected with the lower end part ofthe section 553A, particularly the right side in FIG. 21 or a sideextending along the first direction 51 of the lower end part of thesection 553A. The right end of the section 554A is connected with thelower end part of the section 553B, particularly the left side in FIG.21 or a side extending along the first direction 51 of the lower endpart of the section 553B.

The section 554B extending in the second direction is connected with thesections 553B and 553C extending in the first direction at both ends.The left end of the section 554B is connected with the upper end part ofthe section 553B, particularly the right side of the upper end part ofthe section 553B. The right end of the section 554B is connected withthe upper end part of the section 553C, particularly the left side ofthe upper end part of the section 553C.

In each of the sections 554A and 554B extending in the second direction,a virtual middle line extending straight in the second direction 54 fromone end to the other end can be defined. The position P1 in the firstdirection 51 corresponds to the position of the virtual line VLA of thesection 554A. The virtual line VLA of the section 554A has a length LHA.The position P2 in the first direction 51 corresponds to the position ofthe virtual line VLB of the section 554B extending in the seconddirection. The virtual line VLB of the section 554B has a length LHB.

At each position in the first direction of the virtual line of a sectionextending in the second direction, the sum TL of the length LH* of thevirtual line and the product of the number of sections extending in thefirst direction that exist along the second direction and the channelwidth W is the same value. The asterisk (*) in LH* is a so-calledwildcard representing a null or a character string consisting of one ormore characters.

The product of the number of sections extending in the first directionthat exist along the second direction and the channel width at theposition in the first direction of the virtual line of a sectionextending in the second direction corresponds to the total sum of thechannel widths of the sections extending in the first direction at theposition. The channel width means the dimension in the second directionof the channel and equals to the width (dimension in the directionperpendicular to the first direction) of the so-called channel as theelectric current path of the TFT. It is preferable that the width of theso-called channel as the electric current path of the TFT be uniformthroughout the sections extending in the first direction and thesections extending in the second direction. Specifically, at theposition P1 of the virtual line VLA of the section 554A, there exist twosections 553A and 553B extending in the first direction. The length ofthe virtual line VLA is LHA. Accordingly, the total length TL is 2W+LHA.

At the position P2 of the virtual line VLB of the section 554B, thereexist two sections 553B and 553C extending in the first direction. Thelength of the virtual line VLB is LHB. Accordingly, the total length TLis 2W+LHB. The length LHA is equal to the length LHB. That is to say,the total lengths TL at the positions P1 and P2 are the same value.

At any positions in the first direction where a virtual line extendingstraight in the second direction does not cross any section extending inthe second direction, the products of the number of sections extendingin the first direction that is crossed by the virtual line and thechannel width W are the same value. For example, at a position P3 in thefirst direction 51, there is no section extending in the seconddirection but exist sections 553A, 553B, and 553C extending in the firstdirection. Accordingly, the product of the number of sections extendingin the first direction and the channel width W is 3W. In the example ofFIG. 21, the values of LHA, LHB, and W are the same. That is to say, thetotal lengths TL at positions P1, P2, and P3 are all the same 3W.

The channel ends 556 and 557 defining the channel length L are locatedinner than the upper end and the lower end of the channel 551B definingthe channel size LB2 in the first direction. More specifically, theposition in the first direction 51 of the channel end 556 is the same asthe position of the lower end of the section 554B. Further, the positionin the first direction 51 of the channel end 557 is the same as theposition of the upper end of the section 554A. The lengths in the firstdirection of the sections 553A and 553C are equal. The length in thefirst direction of the section 553B is longer than the length of thesections 553A and 553C extending by the channel width W.

FIG. 22 illustrates another example of the shape of a channel. A channel551C has a uniform channel width W in its straight parts along theelectric current path. The channel length L is defined along the centralpart of the electric current path between the channel ends 566 and 567.The central part of the electric current path includes the center of theelectric current path and a predetermined width from the center. Thechannel 551C consists of sections 563A, 563B, and 563C extending in thefirst direction 51 and sections 564A and 564B extending in the seconddirection 54. The first direction 51 is parallel to the scanningdirection 51 and the second direction is perpendicular to the scanningdirection 51.

All corners of the channel 551B described with reference to FIG. 21 areright angles. In contrast, some corners of the channel 551C have a curve(R) when seen from the top. Specifically, the channel 551C has curvedcorners on the lower left of the section 563A extending in the firstdirection, the lower right and the upper left of the section 563Bextending in the first direction, and the upper right of the section563C extending in the first direction. These corners are outer curvedcorners. The channel 551C further has curved corners on the upper leftand upper right of the section 564A extending in the second directionand the lower left and lower right of the section 564B extending in thesecond direction. These corners are inner curved corners.

Because of these corners, the total length (or the area) in the channel551C is not perfectly uniform at each position in the scanning direction51. However, the configuration described in the following reduces thedifferences in proportion of the regions having differentcharacteristics throughout the channel, so that the total length (or thearea) can be regarded as substantially uniform at each position in thescanning direction 51.

In the channel 551C, the section 563A extending in the first direction,the section 564A extending in the second direction, the section 563Bextending in the first direction, the section 564B extending in thesecond direction, and the section 563C extending in the first directionare connected in this order.

The section 564A extending in the second direction is connected with thesections 563A and 563B extending in the first direction at both ends.The left end of the section 564A is connected with the lower end part ofthe section 563A, particularly the right side or a side extending alongthe first direction 51 of the lower end part of the section 563A. Theright end of the section 564A is connected with the lower end part ofthe section 563B, particularly the left side or a side extending alongthe first direction 51 of the lower end part of the section 563B.

The section 564B extending in the second direction is connected with thesections 563B and 563C extending in the first direction at both ends.The left end of the section 564B is connected with the upper end part ofthe section 563B, particularly the right side of the upper end part ofthe section 563B. The right end of the section 564B is connected withthe upper end part of the section 563C, particularly the left side ofthe upper end part of the section 563C.

In each of the sections 564A and 564B extending in the second direction,a virtual middle line extending straight in the second direction 54 fromone end to the other end can be defined. The position P1 in the firstdirection 51 corresponds to the position of the virtual line VLA of thesection 564A. The virtual line VLA of the section 564A has a length LHA.The position P2 in the first direction 51 corresponds to the position ofthe virtual line VLB of the section 564B. The virtual line VLB of thesection 564B has a length LHB.

At the position P1 of the virtual line VLA of the section 564A, thereexist two sections 563A and 563B extending in the first direction. Thelength of the virtual line VLA is LHA. Accordingly, the total length TLis 2W+LHA. At the position P2 of the virtual line VLB of the section564B, there exist two sections 563B and 563C extending in the firstdirection. The length of the virtual line VLB is LHB. Accordingly, thetotal length TL is 2W+LHB. The length LHA is equal to the length LHB.That is to say, the total lengths TL at the positions P1 and P2 are thesame value.

At a position P3 in the first direction 51, there is no sectionextending in the second direction but exist sections 563A, 563B, and563C extending in the first direction. Accordingly, the product of thenumber of sections extending in the first direction and the channelwidth W is 3W. The values of LHA, LHB, and W are the same. That is tosay, the total lengths TL at the positions P1, P2, and P3 are all thesame 3W. At the other positions in the first direction where a virtualline extending straight in the second direction does not cross anysection extending in the second direction, the products of the number ofsections extending in the first direction that is crossed by the virtualline and the channel width are 3W.

The channel ends 566 and 567 defining the channel length L are locatedinner than the upper end and the lower end of the channel 551C definingthe channel size LB3 in the first direction. More specifically, theposition in the first direction 51 of the channel end 566 is the same asthe position of the lower end of the straight part of the section 564B.The lower end of the straight part of the section 564B is located at theposition defining the channel width W of the section 564B.

Further, the position in the first direction 51 of the channel end 567is the same as the position of the upper end of the straight part of thesection 564A. The upper end of the section 564A is located at theposition defining the channel width W of the section 564A. The lengthsin the first direction of the sections 563A and 563C are equal. Thelength in the first direction of the section 563B is longer than thelength in the first direction of the sections 563A or 563C by thechannel width W.

FIG. 23 illustrates the relation between the channel 551C having curved(R) corners when seen from the top and the channel 551B whose cornersare all right angles. In FIG. 23, the channel 551C is indicated by asolid line and the channel 551B is indicated by a dashed line and theyare superposed one above the other. In FIG. 23, the channel width W andthe channel length L are common to the channels.

As understood from FIG. 23, the shape obtained by virtual end faces(sides) continued straight from the end faces (sides) defining thechannel width W of the channel 551C is identical to the shape of thechannel 551B. Specifically, the end faces 571 and 572 extending in thefirst direction 51 are extended downward. The end faces 573, 574, 577,and 578 extending in the second direction 54 are extended both leftwardand rightward. The end faces 575 and 576 extending in the firstdirection 51 are extended both upward and downward. The end faces 579and 580 extending in the first direction are extended upward.

The shape surrounded by the virtual end faces formed as described aboveis identical to the channel shape of the channel 551B. That is to say,regarding this virtual shape, the total length (or the area) isperfectly uniform at each position in the scanning direction (firstdirection) 51. In other words, the areas of the parts of this virtualshape sectioned in the scanning direction (first direction) 51 areperfectly uniform. The channel 551C having such a shape can effectivelyreduce the differences in proportion of the regions having differentcharacteristics throughout the channel.

FIG. 24 illustrates still another example of the shape of a channel. Achannel 551D includes more sections extending in the first direction andmore sections extending in the second direction than the above-describedchannels 551B or 551C. In the channel 551D, all corners are right angleslike those in the channel 551B and the total length (area) is perfectlyuniform at each position in the scanning direction (first direction) 51.

The channel 551D has a uniform channel width W. The channel length L isdefined between the channel ends 586 and 587. The channel 551D consistsof sections 583A to 583D extending in the first direction 51 andsections 584A to 584E extending in the second direction 54. The firstdirection 51 is parallel to the scanning direction 51 and the seconddirection 54 is perpendicular to the scanning direction 51.

In the channel 551D, the section 584A extending in the second direction,the section 583A extending in the first direction, the section 584Bextending in the second direction, the section 583B extending in thefirst direction, the section 584C extending in the second direction, thesection 583C extending in the first direction, the section 584Dextending in the second direction, the section 583D extending in thefirst direction, and the section 584E extending in the second directionare connected in this order.

The section 584A extending in the second direction is connected with asection extending in the first direction only at one end. Specifically,the right end of the section 584A is connected with the lower end partof the section 583A, particularly the left side or a side extendingalong the first direction 51 of the lower end part of the section 583A.

The section 584B extending in the second direction is connected with thesections 583A and 583B extending in the first direction at both ends.The left end of the section 584B is connected with the upper end part ofthe section 583A, particularly the right side of the upper end part ofthe section 583A. The right end of the section 584B is connected withthe upper end part of the section 583B, particularly the left side ofthe upper end part of the section 583B.

The section 584C extending in the second direction is connected with thesections 583B and 583C extending in the first direction at both ends.The left end of the section 584C is connected with the lower end part ofthe section 583B, particularly the right side of the lower end part ofthe section 583B. The right end of the section 584C is connected withthe lower end part of the section 583C, particularly the left side ofthe lower end part of the section 583C.

The section 584D extending in the second direction is connected with thesections 583C and 583D extending in the first direction at both ends.The left end of the section 584D is connected with the upper end part ofthe section 583C, particularly the right side of the upper end part ofthe section 583C. The right end of the section 584D is connected withthe upper end part of the section 583D, particularly the left side ofthe upper end part of the section 583D.

The section 584E extending in the second direction is connected with asection extending in the first direction only at one end. Specifically,the left end of the section 584E is connected with the lower end part ofthe section 583D, particularly the right side or a side extending alongthe first direction 51 of the lower end part of the section 583D.

In each of the sections 584A to 554E extending in the second direction,a virtual middle line extending straight in the second direction 54 fromone end to the other end can be defined. The position P1 in the firstdirection 51 corresponds to the position of the virtual line VLA of thesection 584A. The virtual line VLA of the section 584A has a length LHA.

The position P2 in the first direction 51 corresponds to the position ofthe virtual line VLB of the section 584B. The virtual line VLB of thesection 584B has a length LHB. The position P3 in the first direction 51corresponds to the position of the virtual line VLC of the section 584C.The virtual line VLC of the section 584C has a length LHC. The positionP4 in the first direction 51 corresponds to the position of the virtualline VLD of the section 584D. The virtual line VLD of the section 584Dhas a length LHD. The position of the virtual line VLE of the section584E is the same position P3 as the virtual line VLC. The virtual lineVLE of the section 584E has a length LHE.

At the position P1 of the virtual line VLA of the section 584A, thereexist four sections 583A to 583D extending in the first direction. Thelength of the virtual line VLA is LHA. Accordingly, the total length TLis 4W+LHA. At the position P2 of the virtual line VLB of the section584B, there exist four sections 583A to 583D extending in the firstdirection. The length of the virtual line VLB is LHB. Accordingly, thetotal length TL is 4W+LHB.

At the position P3, there exist two sections 584C and 584E extending inthe second direction and three sections 583B, 583C, and 583D extendingin the first direction. The lengths of the virtual lines VLC and VLE areLHC and LHE, respectively. Accordingly, the total length TL is3W+LHC+LHE. At the position P4 of the virtual line VLD of the section584D, there exist two sections 583C and 583D extending in the firstdirection. The length of the virtual line VLD is LHD. Accordingly, thetotal length TL is 2W+LHD.

In the channel 551D in FIG. 24, the lengths LHA, LHB, LHC, and LHE areequal to the channel width W. The length LHD is three times of thechannel W. Accordingly, the total lengths TL at all positions P1 to P4are the same 5W. Meanwhile, a virtual line extending in the seconddirection at any position in the first direction 51 overlaps at leastone section extending in the second direction in the channel 551D. Inother words, when the channel 551D is seen in the second direction 54,there is at least one section extending in the second direction at anyposition in the first direction 51.

Returning to FIG. 20, the channel size LB2 of the channel 551B in thescanning direction 51 is an integral multiple of the scanning pitchP_(ELA) of the pulse laser beam 50. This configuration enables thechannel 551B to exhibit the same channel characteristics when thechannel 551B is located at any phase of an irradiation cycle in thescanning direction.

As described above, the channel 551B has a uniform total length TL ateach position in the scanning direction 51. Accordingly, when thechannel size LB2 in the scanning direction 51 of the channel 551B is anintegral multiple of the scanning pitch P_(ELA), the channel 551B hasthe same size of area at every phase. For this reason, the proportionsof the regions having different characteristics are the same among aplurality of channels 551B. Accordingly, when the channel size LB2 is anintegral multiple of the scanning pitch P_(ELA), uniform channelcharacteristics can be maintained, regardless of the position of thechannel 551B.

The same applies to the bending channel 551D described with reference toFIG. 24. As to the channel 551C described with reference to FIG. 22, thetotal length (area) at each position in the scanning direction 51 is notperfectly uniform since the channel 551C have curved (R) corners whenviewed from the top, as described above. However, the channel 551Dconfigured as described above can show the uniform channelcharacteristics independently from the position where the channel 551Dis located, if the channel size LB4 of the channel 551D is an integralmultiple of the scanning pitch P_(ELA).

In another configuration example, channels 551B, 551C, or 551D havingthe above-described bending shape are configured to have a channel sizeLB of an integral multiple of the scanning pitch P_(ELA) and disposed atthe same phase in ELA irradiation cycles. As a result, thecharacteristics become more uniform among the channels.

In still another configuration example, channels 551B, 551C, or 551Dhaving the above-described bending shape are configured to have achannel size LB different from an integral multiple of the scanningpitch P_(ELA) and disposed at the same phase in ELA irradiation cycles.

In Embodiment 2, the channels of the driving TFTs for the pixels of thesame color are provided at the same phase in ELA irradiation cycles.Accordingly, channels having a desirably bent shape can have uniformchannel characteristics, independently from their locations. However,the actual channel locations could be fluctuated with respect to (differfrom) the designed positions. When each channel has a bending shape thatexhibits small differences in proportion of the regions having differentcharacteristics throughout the channel, the differences incharacteristics among a plurality of channels can be made small even ifthe channels are slightly dislocated from the designed positions. Such abending channel can have a channel size different from an integralmultiple of the scanning pitch P_(ELA).

Now, the reason for the description about FIG. 20, why thecharacteristics of a transistor having a bending channel can bedescribed as the synthetic characteristics of the individualphase-related characteristics of the channel portions, independentlyfrom the direction of the electric current, is described.

An experiment was conducted with closely-spaced test transistors eachhaving a channel width and a channel length both of W. This Wcorresponds to the W described in FIG. 20. The test transistors includetwo types of transistors: parallel type of test transistors(hereinafter, referred to as parallel type) and perpendicular type oftest transistors (hereinafter, referred to as perpendicular type). Theparallel type is a transistor disposed in such a manner that thedirection of the channel length or the direction of electric current isparallel to the scanning direction. On the other hand, the perpendiculartype is a transistor disposed in such a manner that the direction of thechannel length or the direction of electric current is perpendicular tothe scanning direction.

A plurality of pairs each consisting of one parallel type and oneperpendicular type were prepared and disposed on a substrate. In eachpair, a parallel type and a perpendicular type are disposed closely. Inthe plurality of pairs, each parallel type and each perpendicular typeare disposed at slightly different positions, particularly at differentphases in irradiation cycles of the pulse laser beam.

The characteristics of the test transistors disposed in such a mannerwere measured, sorted by the phases where the transistors are located,and made comparison between the parallel type and the perpendiculartype. The result of the comparison revealed that the characteristics areequal and further, the variations of the characteristics with phase arealso equal. That is to say, as to a transistor having a channel widthsuitable for a pixel circuit, individual channel portions (or fragments)obtained by dividing the channel along the channel length into portionshaving a length equal to a channel width have the same (or substantiallyequal) characteristics no matter whether the electric current flows inparallel to or perpendicularly to the ELA scanning direction

Accordingly, as described with reference to FIG. 20, the characteristicsof a transistor having a bending channel can be described as thesynthetic characteristics of the individual phase-relatedcharacteristics of the channel portions.

FIGS. 21 to 24 have provided examples where the angle between the firstdirection of each section extending in the first direction and thescanning direction is 0 degrees, in other words, the first direction isparallel to the scanning direction. However, the absolute value betweenthe first direction and the scanning direction can be more than 0degrees. FIGS. 25A to 25C illustrate examples of the channel shape wherethe angles between the first direction and the scanning direction are 3degrees, 10 degrees, and 20 degrees. In FIG. 25A, the absolute values ofthe angles between the first direction d11 and the scanning direction ofsections 593A, 593B, and 593C extending in the first direction in achannel 591B are 3 degrees. The two-dot chain line 51 a is a lineparallel to the scanning direction 51. The first direction d11 isindicated by a dotted line in FIG. 25A and is along the long side ofeach section extending in the first direction.

In the channel 591B, the sections extending in the first direction andthe sections extending in the second direction are connectedalternately. Specifically, the section 593A extending in the firstdirection, the section 594A extending in the second direction, thesection 593B extending in the first direction, the section 594Bextending in the second direction, and the section 593C extending in thefirst direction are connected in this order. Since this connection isdescribed with FIG. 21, description about this connection is omittedhere.

In FIG. 25B, the absolute values of the angles between the firstdirection d111 and the scanning direction of sections 603A, 603B, and603C extending in the first direction in a channel 601B are 10 degrees.The first direction d111 is indicated by a dotted line in FIG. 25B andis along the long side of each section extending in the first direction.In the channel 601B, the sections extending in the first direction andthe sections extending in the second direction are connectedalternately. Specifically, the section 603A extending in the firstdirection, the section 604A extending in the second direction, thesection 603B extending in the first direction, the section 604Bextending in the second direction, and the section 603C extending in thefirst direction are connected in this order. Since this connection isdescribed with FIG. 21, description about this connection is omittedhere.

In FIG. 25C, the absolute values of the angles between the firstdirection d1111 and the scanning direction of sections 613A, 613B, and613C extending in the first direction in a channel 611B are 20 degrees.The first direction d1111 is indicated by a dotted line in FIG. 25C andis along the long side of each section extending in the first direction.In the channel 611B, the sections extending in the first direction andthe sections extending in the second direction are connectedalternately. Specifically, the section 613A extending in the firstdirection, the section 614A extending in the second direction, thesection 613B extending in the first direction, the section 614Bextending in the second direction, and the section 613C extending in thefirst direction are connected in this order. Since this connection isdescribed with FIG. 21, description about this connection is omittedhere.

Generalizing the angle between the first direction and the scanningdirection, the absolute value of the angle between the first directionand the scanning direction is a predetermined angle. Preferably, thepredetermined angle is 0 degrees. As far as the effects described in theembodiments of this disclosure are attained, the predetermined angle canbe more than 0 degrees and not more than 20 degrees, for example.

The dimension (size) along the first direction of the channel and thedimension along the scanning direction of a channel satisfy thefollowing formula:

D2=D1×cosθ,

where D2 represents the dimension along the scanning direction of thechannel, D1 represents the dimension along the first direction of thechannel, and θ represents the predetermined angle. The dimension alongthe scanning direction of the channel is an integral multiple of thescanning pitch of a pulse laser beam.

In the cases where the absolute value of the aforementioned angle ismore than 0 degrees, the product of the number of sections extending inthe first direction that exist along the second direction and thechannel width at the position in the first direction of the virtual lineof a section extending in the second direction is the total sum of thechannel widths of the sections extending in the first direction. Thechannel width of each section extending in the first direction is equalto the dimension along the second direction of the channel but is notalways equal to the width (the width in the direction perpendicular tothe first direction) of the so-called channel as the current path of theTFT.

As set forth above, embodiments of this disclosure have been described;however, this disclosure is not limited to the foregoing embodiments.Those skilled in the art can easily modify, add, or convert each elementin the foregoing embodiments within the scope of this disclosure. A partof the configuration of one embodiment can be replaced with aconfiguration of another embodiment or a configuration of an embodimentcan be incorporated into a configuration of another embodiment.

What is claimed is:
 1. A display device comprising: a substrate; aplurality of light-emitting elements on the substrate; and a pluralityof pixel circuits on the substrate, being configured to control theplurality of light-emitting elements in one-to-one correspondence,wherein each of the plurality of pixel circuits includes a thin filmtransistor, wherein the thin film transistor includes a channel; whereinthe plurality of pixel circuits are disposed at different positions in ascanning direction of a pulse laser beam for annealing the channels, andwherein at least channels for light-emitting elements of the same colorout of the channels are disposed at the same phase of irradiation cyclesof the pulse laser beam in the scanning direction.
 2. The display deviceaccording to claim 1, wherein the channels are made of poly-silicon, andwherein the channels are poly-crystallized by being annealed.
 3. Thedisplay device according to claim 1, wherein the thin film transistor isa driving thin film transistor configured to control electric current tobe supplied to the light-emitting element to control light emission ofthe light-emitting element.
 4. The display device according to claim 1,wherein the plurality of pixel circuits include pixel circuitssuccessively disposed in the scanning direction, wherein N times of apixel circuit pitch in the scanning direction of the successivelydisposed pixel circuits is equal to M times of a scanning pitch of thepulse laser beam, wherein the N is a natural number greater than 1 andthe M is a natural number greater than the N, wherein each of thesuccessively disposed pixel circuits includes a transistor including achannel, wherein the channels of the successively disposed pixelcircuits are disposed at the same phase of irradiation cycles of thepulse laser beam in the scanning direction, and wherein a pattern oflocations of channels in a pixel circuit unit consisting of successive Npixel circuits is repeated in the successively disposed pixel circuits.5. The display device according to claim 4, wherein, in the pixelcircuit unit, distances between channels adjacent to each other areintegral multiples of the scanning pitch of the pulse laser beam, andwherein the largest difference values among the distances in the pixelcircuit unit is equal to the scanning pitch.
 6. The display deviceaccording to claim 1, wherein the channel is composed of sectionsextending in a first direction that is parallel to the scanningdirection and sections extending in a second direction that isperpendicular to the scanning direction and is formed by connecting thesections extending in the first direction and the sections extending inthe second direction alternately, wherein at least one end of eachsection extending in the second direction is connected with a sideextending along the first direction of an end part in the firstdirection of a section extending in the first direction, wherein, ineach section extending in the second direction, a middle line extendingstraight in the second direction from one end to the other end isdefined as first virtual line, wherein, at each position in the firstdirection of the first virtual lines, a sum of a total length of thefirst virtual lines and a product of a channel width and a number ofsections extending in the first direction that are arranged in thesecond direction, takes the same value, and wherein, at a position inthe first direction where a second virtual line extending straight inthe second direction does not cross any section extending in the seconddirection, a product of a number of sections extending in the firstdirection that is crossed by the second virtual line and the channelwidth takes a value equal to the same value.
 7. A display devicecomprising: a substrate; a plurality of light-emitting elements on thesubstrate; and a plurality of pixel circuits on the substrate, beingconfigured to control the plurality of light-emitting elements inone-to-one correspondence, wherein each of the plurality of pixelcircuits includes a thin film transistor, wherein the thin filmtransistor includes a channel; wherein the channel is composed ofsections extending in a first direction and sections extending in asecond direction, an absolute value of an angle between the firstdirection and a scanning direction of a pulse laser beam for annealingthe channel is a predetermined value, the second direction isperpendicular to the scanning direction, and the sections extending inthe first direction and sections extending in the second direction areconnected alternately, wherein at least one end of each sectionextending in the second direction is connected with a side extendingalong the first direction of an end part in the first direction of asection extending in the first direction, wherein, in each sectionextending in the second direction, a middle line extending straight inthe second direction from one end to the other end is defined as firstvirtual line, wherein, at each position in the first direction of thefirst virtual lines, a sum of a total length of the first virtual linesand a product of a number of sections extending in the first directionthat exist in the second direction and a channel width takes the samevalue, wherein, at a position in the first direction where a secondvirtual line extending straight in the second direction does not crossany section extending in the second direction, a product of a number ofsections extending in the first direction that is crossed by the secondvirtual line and the channel width takes a value equal to the samevalue, and wherein a dimension in the scanning direction of the channelis an integral multiple of a scanning pitch of the pulse laser beam. 8.The display device according to claim 7, wherein the channel consistsof: a first section extending in the first direction; a second sectionextending in the second direction connected with the first section; athird section extending in the first direction connected with the secondsection; a fourth section extending in the second direction connectedwith the third section; and a fifth section extending in the firstdirection connected with the fourth section.
 9. The display deviceaccording to claim 7, wherein sums of lengths of overlap segments ofthird virtual lines extending straight in the second direction atdifferent positions in the first direction with the channel are the samevalue.
 10. The display device according to claim 7, wherein a part of asection extending in the first direction where the section extending inthe first direction is connected with a section extending in the seconddirection includes a curve.
 11. The display device according to claim10, wherein third virtual lines extending straight in the seconddirection are located at different positions in the first direction butoutside the curve, and wherein sums of lengths of overlap segments ofthe third virtual lines with the channel are the same value.
 12. Thedisplay device according to claim 7, wherein the plurality of pixelcircuits are disposed at different positions in the scanning direction,and wherein at least channels for light-emitting elements of the samecolor out of the channels are disposed at the same phase of irradiationcycles of the pulse laser beam in the scanning direction.
 13. Thedisplay device according to claim 7, wherein a dimension along the firstdirection of the channel and a dimension along the scanning direction ofthe channel satisfy the following formula:D2=D1×cosθ, where D2 represents the dimension along the scanningdirection of the channel, D1 represents the dimension along the firstdirection of the channel, and θ represents the predetermined angle.